Fix comment about cross-checking the varnullingrels
The nullingrels match checks are not limited to debugging builds.
Oversight in commit 867be9c07.
Author: Richard Guo
Reviewed-by: Alvaro Herrera, Tom Lane, Robert Haas
Discussion:
Fix RBM_ZERO_AND_LOCK.
Commit 210622c6 accidentally zeroed out pages even if they were found in
the buffer pool. It should always lock the page, but it should only
zero pages that were not already valid. Otherwise, concurrent readers
that hold only a pin could see corrupted page contents
